Big Data Study: 1 in 25 Given Death Penalty Sentence Are Likely Innocent

A view of the death chamber from the witness room at the Southern Ohio Correctional Facility.

Photo by Mike Simons/Getty Images

A new study published online this week by the National Academy of Sciences takes a shot at determining the rate at which the U.S. mistakenly sentences innocent prisoners to death. The findings are unsettling. The study’s authors conclude that based on the statistical data, it can safely be estimated that 4.1 percent, or one-in-25 criminal defendants, sentenced to death in the U.S. are innocent. In fact, that’s probably low-balling the actual number of erroneous death penalty sentences. “We conclude that this is a conservative estimate of the proportion of false conviction among death sentences in the United States,” the study’s abstract reads.

The rate of erroneous conviction of innocent criminal defendants is often described as not merely unknown but unknowable. There is no systematic method to determine the accuracy of a criminal conviction; if there were, these errors would not occur in the first place. As a result, very few false convictions are ever discovered, and those that are discovered are not representative of the group as a whole. In the United States, however, a high proportion of false convictions that do come to light and produce exonerations are concentrated among the tiny minority of cases in which defendants are sentenced to death. This makes it possible to use data on death row exonerations to estimate the overall rate of false conviction among death sentences.

“From 1973 to 2004, 1.6 percent of those sentenced to death in the U.S. — 138 prisoners — were exonerated and released because of innocence,” the Associated Press reports. That number, however, according to the study, likely short changes the actual number of wrongly handed down death sentences for a simple reason—while on death row the inmates’ cases receive a much higher level of scrutiny. That leads the authors to surmise, the actual number of mistaken executions is much lower than the number of those sentenced to death.

Here’s more on that from the AP:

[T]he great majority of innocent people who are sentenced to death are never identified and freed, says professor Samuel Gross of the University of Michigan Law School, the study’s lead author. The difficulty in identifying innocent inmates stems from the fact that more than 60 percent of prisoners in death penalty cases ultimately are removed from death row and resentenced to life imprisonment. Once that happens, their cases no longer receive the exhaustive reviews that the legal system provides for those on death row… The research produced an estimate of the percentage of defendants who would be exonerated if they all remained indefinitely on death row, where their cases would be subject to intense scrutiny for innocence… The study concluded that the number of innocent defendants who have been put to death is “comparatively low. … Our data and the experience of practitioners in the field both indicate that the criminal justice system goes to far greater lengths to avoid executing innocent defendants than to prevent them from remaining in prison indefinitely.”

What does that mean exactly? Here’s the takeaway from the study: “most innocent defendants who have been sentenced to death have not been exonerated, and many — including the great majority of those who have been resentenced to life in prison — probably never will be.”